Abstract
Measurements of inclusive four-lepton ($4\ell$) production in $\sqrt{s} = 13$~TeV proton-proton collisions at the Large Hadron Collider using the ATLAS Run-2 (2015-2018) dataset of $139$~fb$^{-1}$ are presented. The $4\ell$ final state, consisting of two same-flavor, opposite-charge lepton pairs, exhibits a rich structure with contributions from several modes of production - including $Z$-boson and Higgs resonant decays and a variety of multi-boson processes - and benefits from a clean detector signature, making it the ideal candidate for both precision studies of the Standard Model and interpretations of new physics beyond-the-Standard Model. Four-lepton production cross-sections are measured inclusively in several invariant mass regions and differentially versus a variety of other kinematic observables. The measurements are corrected for detector effects and compared to state-of-the-art Standard Model predictions, which are found to be consistent with the data. The most precise measurement to date of the $Z \to 4\ell$ branching fraction is extracted and found to be in agreement with prior measurements and theoretical predictions. Constraints are placed on two scenarios of new physics, while further re-interpretations remain promising given the inclusive nature of the analysis and minimal model dependence in the measurements.